The encapsulation of viable cells which produce biologically-active factors has experienced substantial growth and increased interest in recent years. These special implantable, encapsulating devices are capable of providing a vast array of biological functions and services. For example, biologically active therapeutic agents of living cells, such as enzymes, neurotransmitters, blood coagulation factors, lymphokines, cytokines, nerve growth factors, trophic factors such as neurotrophic factor, hormones and angiogenesis factors, may be continuously diffused into a host for therapeutic purposes. In other instances, these agents may be employed for diagnostic purposes. For example, the implanted cells could react to excrete some measurable product or the like in response to a particular physiological condition.
After considerable research, two general encapsulation approaches have evolved. One approach involves the manufacture of an encapsulating membrane around the viable cell cultures. Usually, microcapsules or microspheres, encapsulating a microscopic droplet of cell solution, are provided which are integral structures not generally requiring post-production sealing. This approach is disclosed in U.S. Pat. Nos. 4,353,888 to Sefton and 4,352,833 to Lim; and European Patent No. 188,309 to Rha. One problem with these devices is that they are limited in volume, difficult to manufacture, implant and retrieve, and often suffer from limited biocompatibility.
Another encapsulation approach involves the use of macroencapsulation devices defining a cell suspension reservoir or lumen formed to hold the cell culture solution therein. These devices provide a much greater cell solution volume and are substantially easier to handle in both implantation and retrieval. One technique of fabricating a macroencapsulating device involves the coextrusion of an aqueous cell culture and a polymeric solution which forms a tubular extrudate having a polymeric outer coating encapsulating the viable cell solution. In some instances, the cell culture is fully encapsulated during the integral fabrication thereof, while in other instances, post-production sealing of the lumen is required. Examples of these coextrusion devices may be found in U.S. Pat. No. 5,158,881 to Aebischer et al.
Another macrocapsule fabrication technique includes providing an elongated hollow fiber macroencapsulation structure which is subsequently loaded with the implantable cell cultures. In this approach, the hollow fiber macrocapsule is fabricated with one or more openings to the cell solution reservoir or lumen for cell loading, which subsequently must be sealed to fully encapsulate the cell cultures. Examples of these devices may be found in U.S. Pat. No. 3,615,024 to Michaels.
Flatsheet encapsulation devices are also employed which generally include two flatsheet membranes encapsulating the cells therebetween to form an encapsulating sandwich. Both the cylindrical hollow fiber configuration and the flatsheet configuration provide a more favorable ratio (as compared to a sphere) between the surface area of the membrane and the volume of encapsulated tissue. In macrocapsules of these shapes, as the volume of the device is increased in order to contain greater amounts of encapsulated tissue, the corresponding surface area of the membrane increases more proportionately such that the diffusional transport of nutrients and products for increased amounts of tissue can be accommodated by increasing the surface area without unwieldy increases in total vehicle size.
These encapsulating membrane devices are generally comprised of thermoplastic polymer or copolymer membranes which exhibit characteristics of water insolubility and biocompatibility. This membrane material must be permselective to select therapeutic agents and cell nutrients, yet be impermeable to the cells producing those agents. Upon deposition or loading of the culture solution in the lumen of the hollow fiber, moisture infiltrates throughout the membrane and becomes trapped in the pores. Accordingly, the inner surface wall of the fiber defining the opening into the lumen becomes "wet" regardless of whether or not there has been direct contact with any of the aqueous cell solution. Hence, "wet" sealing techniques must be applied to seal the loading openings. The nature of the pores are such that moisture is drawn in by capillary action. In the case of narrow diameter fiber devices, capillary action within the fiber lumen further serves to distribute water and contaminants throughout the length of the fiber.
Traditional approaches to wet sealing thermoplastic encapsulation devices include the employment of polymer adhesives and/or crimping, knotting and heat sealing. Examples of these wet sealing techniques may be found in the following publications: J. Altman et al., "Successful Pancreatic Xenografts Using Semipermeable Membrane", 5 Artificial Organs (Suppl.) 776 (1981) (Polyvinylchloride acrylic XM50 copolymer tubing biocompatible epoxy or cyacrylate glue); J. Altman et al., "Long-Term Plasma Glucose Normalization in Experimental Diabetic Rats With Macroencapsulated Implants of Benign Human Insulinomas", 35 Diabetes 625, (1986) (poly(acrylonitrile-co-vinyl-chloride) (PAN/PVC) copolymer glue in solvent); B. Dupuy et al., "In Situ Polymerization of a Microencapsulating Medium Round Living Cells", 22 J. Biomed. Materials Res. 1061 (1988) (Photopolymerization of membranes around cells): W. Hymer et al., "Pituitary Hollow fiber Units In Vivo and In Vitro", 32 Neuroendocrinology 33 9 (1981) (PAN/PVC fibers syringe loaded, crimping with heated forceps); H. Iwata et al., "The Use of Photocrosslinkable Polyvinyl Alcohol in the Immunoisolation of Pancreatic Islets", 22 Transplant Proceedings 797 (April 1990) (Production of encapsulated cells using photocrosslinkable hydrogel); Y. Kojima et al., "Xenogeneic Pancreatic Islet Transplantation Using a Millipore Diffusion Chamber", 19 Transplant Proceedings 981 (February 1987) (Millipore MF cement); P. Lamberton et al., "Use of Semipermeable Polyurethane Hollow Fibers for Pituitary Organ Culture", 24 In vitro Cellular & Developmental Biology 500 (June 1988); C. Lum et al., "Intraperitoneal Nucleopore Chambers: a Murine Model for Allograft Rejection", 20 Transplant Proceedings 173 (April 1988) (Nucleopore membranes attached with silicone sealant; Millipore MF cement); S. Ronel et al., "Macroporous Hydrogel Membranes for a Hybrid Artificial Pancreas", 17 J. Biomed. Materials Res. 855 (1983) (Pressure/heat sealing of hydrogel encapsulation devices); N. Theodorou et al., "Problems in the Use of Polycarbonate Diffusion Chambers for Syngeneic Pancreatic Islet Transplantation in Rats", 18 Diabetologia 313 (1980) (Polycarbonate filters sealed with polyacrylic cement); F. Wong et al., "Effects of Thymus Enclosed in Millipore Diffusion Envelopes on Thymectomized Hamsters", 28 Blood 40 (1966); and G. Zondervan et al., "Design of a Polyurethane Membrane for the Encapsulation of Islets of Langerhans", 13 Biomaterials 136 (1992) (Polyurethane tubing sealed by knotting).
While these conventional methods of "wet" sealing may be adequate for laboratory experimentation or for short-term usage, their long-term performance has often been inconsistent or unreliable. Potentially, these devices may be implanted in their host for months or years. The nature of the fiber membrane material, to be discussed henceforth, the seal is often breached following implantation. This problem occurs on a consistent basis even when the method of sealing involves the same polymer solvent pair that was used to manufacture the encapsulating device.
Because of the porous nature of the membrane fiber material, moisture, cells, protein, polymers or the like contained in the cell culture solution become trapped in the pores of the membrane. As mentioned, the inner surface wall of the fiber defining the opening into the lumen becomes "wet" regardless of whether there is direct contact with the aqueous cell solution. Most common adhesives for this application, e.g., urethanes or thermoplastic adhesives, such as a PAN/PVC dissolved in the water-miscible solvent dimethylsulfoxide (DMSO), require relatively dry membranes to form a suitable seal and bond. In one instance, exposure to the moisture causes the thermoplastic adhesive to precipitate, thereby preventing adequate bonding to the wall of the fiber. In another instance, both protein and polymers present in the cell culture solution compete with the fiber present for gluing sites resulting in a contamination of the adhesive; thus preventing effective cross-linking in some areas. Hence, seal integrity is substantially degraded.
On the other hand, mechanical deformation (i.e., crimping or knotting), as well as heat sealing, tend to substantially weaken or crack the membrane over time. Due to the relative fragility of the membrane material, even a slight shearing force may fracture the membrane and render the device useless.